A polyester material having an ester bond in its main chain is a linear polymer represented by polyethylene terephthalate and is high in the initial modulus, excellent in the dimensional stability under heating and in the cost, so that it is very useful as a reinforcement for rubber articles such as tire, belt, air spring, rubber hose and the like in forms of filament, cord, cable, cord fabric, canvas and the like.
However, when the polyester material is used as a reinforcement for such rubber articles, since the polyester material is minute in the molecular structure and less in the number of functional groups, adhesion to rubber to rubber is insufficiently obtained by an adhesive composition, effective for adhering materials such as nylon rayon or the like and rubber, comprising a resorcin-formaldehyde condensate obtained by a resol-forming reaction and a rubber latex including vinylpyridine-styrene-butadiene terpolymer.
For this end, there have formerly and numerously been proposed a method wherein surface of a polyester cord is subjected to an alkali-amine treatment to increase the number of an active group such as --OH, --COOH, --NH.sub.2 or the like on the surface, a method wherein a compound having a functional group which is capable of a hydrogen bonding or a primary bonding is introduced onto the surface of the polyester cord using an isocyanate compound or an epoxy compound and then treated with an adhesive solution, and such adhesive compositions.
However, the strength of the polyester cord is degraded by the alkali-amine treatment.
Further, the isocyanate compound or the epoxy compound is high in reactivity and reacts with water as a solvent of the adhesive solution or resorcin-formaldehyde condensate in the adhesive solution, so that it is very difficult to provide an adhesive solution of one package composition and also the adhesion property is damaged. For this end, it is required to divide the adhesion treating step into two stages and hence extra installations and steps are necessary, which are unfavorable from a viewpoint of resource-saving and energy-saving.
Also, the treatment with the isocyanate compound or the epoxy compound is unfavorable from a viewpoint of environmental conservation because of the scattering of these compounds into air during the adhesion treatment, and of injurious effect of fumes and steam created during the heat treatment upon humans, and the like.
Moreover, the polyester cord treated with the isocyanate compound or the epoxy compound is hardened and the handling thereof is difficult in production. In addition, there is a most serious problem that when such a polyester cord is used at a high temperature under a high strain, there are caused rapid deterioration of adhesion and degradation of cord strength. Hence the service life of an rubber article using such a cord as a reinforcing material lowers considerably.
As reasons of these phenomena, there can be mentioned facts that since the isocyanate compound or the epoxy compound is primarily bonded to the polyester surface to thereby form a kind of graft polymer on the surface, the adhesion breakage of the bonded portion is easily caused by hydrolysis of polyester, aminolysis with a residue of vulcanizing agent in a rubber composition and the like in use at a high temperature, and that the bonded portion is weak to mechanical input due to the large difference of rigidity between surface and inside of the polyester under high strain, and the like.
For the weight reduction has been proceeded from a viewpoint of energy-saving, and hence it is necessary to further improve the stability and fatigue life to thermal input and mechanical input as compared with the conventional ones. From this reason, the use of an isocyanate compound or an epoxy compound is also unfavorable in the treatment for the polyester material.
On the other hand, a method of adding a resin imparting adhesion/tackiness as an adhesion promoter to the above-mentioned adhesive solution and adhesive compositions thereof are numerously proposed as an adhesive for the polyester. They enable the adhesion work at one-package type and are excellent in the energy-saving and cost-saving and also the lowering of strength of a cord treated with such an adhesive is small.
Because water having good operability and properties to environment is mainly used as a solvent, there are numerously proposed resins easily soluble in water such as phenolic resin and the like as an adhesion/tackiness imparting resin added. For example, when using a resin obtained by a novolak-forming reaction between resorcin and formaldehyde in the presence of a neutral or acidic catalyst, the adhesion force is improved. But if rubber latex defined in the invention as mentioned below is not used in the adhesive solution, the adhesion performances are insufficient and particularly the adhesion deterioration is caused by thermal input during vulcanization step, when the adhesion to adherend rubber should be enhanced, and it can not be said that the above resin is satisfactory.
Furthermore, there has been proposed, for example, a reaction product between p-chlorophenol and formaldehyde, a reaction product between resorcin and triallyl cyanurate and the like. They are, however, insufficient in the adhesion performances and cause an adhesion deterioration particularly at a higher temperature, so that it can not be said that they are satisfactory.
Recently, in pneumatic tires provided with rubber made members such as carcass and the like using polyester cords as a reinforcing material, since tire performances are improved and a temperature at vulcanization step is made higher than the conventionally used temperature to obtain better productivity, and hence thermal input to the polyester cord becomes much severer over a period ranging from this vulcanization step up to use, there is a possibility that even the aforementioned adhesive compositions become insufficient in heat resistance.
As mentioned above, the properties required in the adhesive composition suitable for use in the polyester material or the like for rubber reinforcement are that it is one-package form, the storing stability is good, it is free from toxicity and environmental pollution, the cord strength is not lowered by the adhesion treatment, the adhesion property is high, the sufficiently high adhesion force at higher temperatures applied during vulcanization step and in use can be maintained, in another words, heat resistance is sufficient, and the like.